Share-Based Compensation Activity Other Than Stock Option (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Service Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period	6,408	10,223	121,113	10,223
Weighted average grant-date fair value per share granted during the period	$ 46.82	$ 35.46	$ 46.80	$ 35.46
Fair value of shares vested during the period	$ 2,104	$ 1,292	$ 2,497	$ 1,740
Performance Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period				34,229
Weighted average grant-date fair value per share granted during the period				$ 33.87
Fair value of shares vested during the period	422	325	1,508	847
Board DSUs And Dividend Equivalents Granted To Board | Former Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period	15,410	21,179	16,181	82,798
Number of DSUs converted to common stock shares during the period			47,189
Weighted average grant-date fair value per share granted during the period	$ 46.82	$ 35.41	$ 46.83	$ 34.31
Fair value of shares vested during the period	$ 1,221	$ 554	$ 1,328	$ 1,624